Man serving extravagant old artist (6)
I believe the answer is:
giotto
'artist' is the definition.
(14th-century Italian painter)
'man serving extravagant old' is the wordplay.
'man serving' becomes 'GI' (GI can mean an American soldier).
'extravagant' becomes 'OTT' (abbreviation for 'over the top').
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'gi'+'ott'+'o'='GIOTTO'
(Other definitions for giotto that I've seen before include "Italian painter, 1267-1337" , "- di Bondone, C 14 Florentine artist" , "C14 Florentine frescoist" , "Fourteenth century Florentine artist" , "Florentine painter" .)